A hydrostatic bearing uses a source of pressurized oil between surfaces to suspend the surfaces from each other to avoid frictional contact. Correct proportioning of the oil pressure and the quantity of the oil flow permits the surfaces to be suspended whether they are moving or not. See volume 2 McGraw-Hill Encyclopedia of Science and Technology, Copyright 1971 pages 127 and 128.
A hydrodynamic bearing relies upon relative motion between surfaces to self generate fluid pressure in fluid disposed between the surfaces to separate the surfaces from each other to avoid frictional contact. See McGraw-Hill Encyclopedia of Science and Technology, Copyright 1971 page 127.
The application of a hydrostatic bearing to a hydraulic motor or pump involves a compromise between leakage and friction, primarily at zero or low rotational speeds. The leakage and friction of a hydrostatic bearing is related to the percent of pressure balance of the bearing. Pressure balance provides a measure of the percentage of the load supported by the pressurized fluid of the hydrostatic bearing. For example, 97% pressure balance means 97% of the load is supported by the pressurized fluid and 3% of the load is supported by the physical bearing structure. A pressure balance of greater than 100% means that the load is more than 100% supported by the film.
One approach in applying a hydrostatic bearing in a hydraulic motor or pump is to use a greater than 100% pressure balance. A greater than 100% pressure balance results in a more rapid formation of a fluid film under all rotational conditions thereby permitting nearly immediate operation of the bearing in a hydrodynamic state. Therefore, extremely low friction values exist at start up and low speeds. However, leakage of the fluid is increased. Increases in leakage of the fluid translates into decreases in moter volumetric efficiency and higher heat production.
The use of less than 100% pressure balance is advantageous in that leakage is reduced, but the resultant increase in breakout and low speed friction may be unacceptable. Typically, a hydrodynamic feature is designed into bearings to support the rotating member by a pressurized fluid film generated by rotation once a high rotating speed is attained.
Hydraulic motors used in devices such as, for example, control surfaces for an Advance Tactical Fighter (ATF) must preload the control surfaces and move the control surfaces at high frequency. A hydraulic motor used in an ATF for moving and preloading the control surfaces is cycled such that it reverses directions numerous times a second, for example, 20 Hz.
Various devices have been proposed for prviding a hydrostatic bearing which is periodically assisted to a greater than 100% pressure balance. One example of such a device is an add on unit having an oil accumulator which stores pressurized oil for use with the crankshaft bearings of an engine. A valve on the accumulator is opened manually to vent the pressurized oil in the accumulator to the crankshaft bearings to obtain a greater than 100% pressure balance prior to and during start up, when the oil pump has not generated sufficient pressure to fully pressurize the crankshaft bearings. This device suffers from various disadvantages, namely that manual operation is required and cannot be used in applications where periodic and rapid initiation of the greater than 100% pressure balance is required.
In electric power generators hydrostatic bearings are provided to reduce friction during start up of the generators.
U.S. Pat. Nos. 611,984, 4,113,325, 4,114,959, and 4,351,574 disclose fluid bearings.